"The Language: Common"

  Because of the confusion of the various languages spoken in the neutral land, and the fascination people had with the humans, most of whom happened to be English, the English language gradually spread.

  It was first a novelty, but because it was a common novelty, it just became a common language, which they came to call Common.

  That world's first introduction to English would be old English, but centuries of further human contact would bring in updated bits and pieces.

  Add to that a natural evolution of Common, and what results is a language that is a mixture of various words and slang from different periods in Earth's speaking history, with some native stuff thrown in.

  It would be very complicated to devise, which is yet another thing keeping the trilogy from being written.

  But it also offers a lot of opportunity for comedy.